(Re-)Structuration of System Dynamics

[Jean-Jacques Laublé]

Hi every one



I have consulted the list of papers presented at the next SD Conference in Oxford.

One has interested me very much at first it is:

>From Reichel, André Number 266 (Re-)Structuration of System Dynamics,



The first part is very interesting for me, summarizing relatively well the difficulties I encounter in trying to model my business problems; difficulty to go from qualitative to quantitative:

For example I have built a qualitative influence diagram that describes very well my problem and which is very readable, understandable and convincing even if it has about 60 variables and about 24 loops something important for my level in SD. But as soon as I try to change it into a quantitative model, everything is going bad. The realism in particular disappears because of the impossibility to determine realistic equations and realistic links between some model variables and reality.

I started back with a one or two loop model (a reinforcing loop with its associated balancing one), built the quantitative model, run it but the problem was the same only simplified:

a difficulty to measure the level and the kind of influence.

So I never developed a quantitative model, trying to get the best from the qualitative one.

I would like to find some intermediate step between full qualitative and full quantitative to

avoid the too high step going from one to the other.

It would then be possible to stay realistic and having some real data to compare to the reality.



In the first part of his paper André Reichel, says that the essence of the behaviour lies in the structure of the model, and that it is possible to draw conclusions only from its structure, avoiding the burden and imprecision (some times) of quantification.

I was of course interested and willing to see how it was possible to do that.



If the rest of the article, there is no proof that the method he proposes may avoid the necessity

of measuring things.

But I would like to clarify one thing:

Is structure the main explanation of the behaviour of a model?



In models of infection diseases, pages 300 to 323 in Business dynamic (to my opinion one of the best demonstration of the power of SD) a small variation of any parameters changes an infection into an epidemic by changing the influence of one loop on another.

For me it proves clearly that structure is not the only responsible of the general model behaviour. For me a model is a structure with its equations. Both are needed.

In an infectious disease the simple structure of the model without its equations, will not tell you anything about its possibility to degenerate into an epidemic.

So I cannot understand how it is possible to build a model by staying qualitative.



Has anybody an idea about this problem?

It is not the first time I read that structure governs behaviour.



Some days ago Geoff Coyle argued:

Some people show a slide of the stock/flow diagram and call it the model. It
isn't, it's a picture of the model and the real model is the equations. As
Jay pointed out, very many years ago, the defence of the model lies in the
defence of its details, that is in the equations, not in the behaviour.



About Geoff Coyle mail; he writes about Professor Pat Rivett's outstanding 'The Craft of Decision Modelling', Wiley 1994.

I could not find the book, nor on Amazon site nor on Wiley's.



Regards to everybody.



J.J. Laublé, Allocar rent a car company

Strasbourg France JEAN-JACQUES.LAUBLE@WANADOO.FR

[reichel@ivr.uni-stuttgart.de]

Hello Jean-Jacques,

in the part you are referring to, I am trying to connect the efforts made in
qualitative system dynamics (QSD) by Geoff Coyle and others to the wider
problem area of theory building in the social sciences. I highly recommend
reading the references given, especially
Coyle, G. 1998. The practice of system dynamics: milestones, lessons and
ideas from 30 years experience. System Dynamics Review 14: 343-365 and
Coyle, G. 2000. Qualitative and quantitative modelling in system dynamics:
some research questions. System Dynamics Review 16: 225-244.
As I heard, Geoff has recently also written a new book about this topic (am
I wrong?). Unfortunately, I have not been able to read it yet.

As for your questions:
""Is structure the main explanation of the behaviour of a model?""
If you are referring to computer models, then that is always the case,
regardless if ""structure"" is truly a structural narrative told in stocks and
flows as in system dynamics or ""just"" a linear function. If it is a mental
model you have in mind, then the answer becomes a bit difficult. Mental
models are described in my paper on pp. 6. and pp. 17.

""So I cannot understand how it is possible to build a model by staying
qualitative.""
You still seem to hold only a quantitative (computer) model to be a model.
That is not the case. Models are virtually everywhere, our entire thinking
is based on models. They are epistemic elements of meaning construction.

As for my paper: I am currently trying to come up with a structural
narrative of the process of structuration in a stock-flow diagramme to make
the re-structuration effort undertaken (and ist benefit!) clearer. Hopefully
this will be finished in early July, when I am starting to prepare my
presentation for the Oxford conference. Cross your fingers that I will not
get lost in translation...

Hope to see you in Oxford
André Reichel
Sender: =?iso-8859-1?Q?Andr=E9_Reichel?= <Andre.Reichel@gmx.de>

____________________________________

André Reichel

University of Stuttgart
Institute of Economics and Law
Department for Environmental and Innovation Research
Keplerstrasse 17
70174 Stuttgart, Germany

Fon: +49 - 711 - 121-3550
Fax: +49 - 711 - 121-2800
E-Mail: reichel@ivr.uni-stuttgart.de
Internet: www.ivr.uni-stuttgart.de/umwelt

[Jean-Jacques Laublé]

Hi André



Thank you for your reply.



I will have to learn a bit more before going further in my understanding of
the problem.

I will read the papers from Geoff Coyle in the SD review and have ordered
his recent book

But it is on strategy and I do not know if it is the subject of the book.

I have tried to find the Craft of decision modelling from Pat Rivett, but I
cannot find it

nor on Amazon or on Wiley. If it has been written in 1994 it should be
possible to find it.



Three remarks:

I have no answer to my question about the epidemic examples given by John
Sterman in his

Business dynamics that show clearly that the structure is not the only
responsible of the behaviour of the model.



Anyhow your work is interesting (only a bit difficult to understand). To
make it clearer you should find some simplistic examples to illustrate your
point.

One is never too simple when explaining something, and in that case one can
always

complicate the subject later on.



A side effect of this discussion is that I have reconsidered the model that
I left to its qualitative state some months ago. One of the reasons of my
difficulties was my extreme pretension about the realism of my equations and
the level of details out of proportion with

the overall model and its objective.



I may see you in Oxford if my schedule will permit it.



Regards



J.J. Laublé Allocar rent a car company
Strasbourg France.
From: =?iso-8859-1?Q?Jean-Jacques_Laubl=E9?= <JEAN-JACQUES.LAUBLE@WANADOO.FR>